Exaptation and vulnerability to introduced mammal herbivores on Balearic endemic flora

Abstract

Aim: Introduced mammal herbivores are predicted to negatively affect insular flora. However, disentangling which particular traits (1) developed from exaptations and (2) are functional to avoid herbivory remains mainly unknown. This study aims to assess if the flora of continental islands with historic native herbivores are exapted to the introduction of new mammal herbivores and to predict the potential vulnerability of endemic species from islands where mammal herbivores have not been introduced. Location: Balearic Islands. Taxon: 96 Balearic endemic plant species. Methods: We investigated whether the endemic flora on continental islands maintains functional traits that resist introduced mammal herbivores by analysing the chemical and morphological traits related to plant resistance of five individuals for each of 98 species. Also, we measured plant-size variables to assess plant escape strategies. Overall, we combined these traits with the accessibility to goats. Predictive models were generated for species that inhabit islands where goats have not been introduced to assess their potential vulnerability. Results: Endemic species may defend against new herbivores (e.g. goats) if they contain highly toxic compounds (alkaloids, glycosides, coumarins), spinescent and urticating structures, or specific plant architecture (low plant size, high specific leaf area). If such traits are absent, the species may become extinct—unless they inhabit areas inaccessible to goats. On continental islands, some endemic species are expected to resist the introduction of herbivores, while others may be significantly affected. Main Conclusions: From the ancient connection with the mainland, exaptations may allow the plants to resist the presence of introduced herbivores. However, non-exapted species could be threatened by the introduction of non-native ungulates